The publication mentioned to Lucas that someone on a local news station “winked at the camera and said there were rumors that Darth Vader (David Prowse, with the voice of James Earl Jones) might not be such a bad guy after all.” As you know, at the film’s end, Darth Vader — who has been evil the entire time — saves his son Luke Skywalker (Mark Hamill) from the Emperor (Ian McDiarmid) and shows Luke his real face. It was a really powerful moment in the theater, and knowing about it going in would have spoiled that moment. Lucas spoke about how it all happened. He said: 

“Well, that was published in a science-fiction magazine, and then they called the newspapers, and the newspapers published what Vader does in the end — the whole end of the movie. They bought a story from one of the crew members in London, and then they put it together with the fact that the title had been changed from ‘Revenge’ to ‘Return.’ They wrote this whole thing about the plot, and that got published in newspapers.”

I cannot imagine the frustration that goes along with spending years crafting a story, which is itself a continuation of years of work for thousands of people, and then having the climax of the tale spoiled before anyone but critics have gotten a chance to see it. I’m still mad about the Luke and Leia (Carrie Fisher) thing, and it’s been decades. One thing about the film did help in that area, though, as Lucas said in the same interview.